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<br />Endangered Fish Population Status Assessment <br /> <br />Introduction <br /> <br />A practical approach is needed for determining the status of or trends in endangered fish populations <br />and whether that status/trend is related to historic or new water development projects. This approach <br />will use measurable population criteria to determine if the Service should (a) reinitiate consultation on <br />water development projects that have been consulted on by the Service or (b) extend the water <br />depletion limit for the upper Colorado River above the 15-mile reach. This approach should utilize the <br />best scientific information available and may be modified in the future based on review of available <br />information and consultations with biostatisticians and other experts, <br /> <br />Assumptions/Limitations <br /> <br />There are a number of practical limitations to our ability to detect changes in the status of endangered <br />fish populations in a timely and accurate manner: <br /> <br />o Colorado pikeminnow (formerly Colorado squawfish): <br />-Adult Colorado pikeminnow can be sampled relatively effectively and reliable <br />population estimates can be developed. Estimates of the size of the adult Colorado <br />pikeminnow population in the Colorado River subbasin will be made every 3-5 years. <br />-Colorado pikeminnow spawning success is monitored annually by sampling for young <br />of the year fish in the fall. The abundance ofYOY fish varies greatly from year to year. <br />-It takes a long time to determine if a year-class of fish will successfully contribute to a <br />population because Colorado pikeminnow do not reproduce in the wild until age 6-7. <br />-Estimates of the numbers of juvenile Colorado pikeminnow (ages 2-5) are unreliable <br />because these life stages cannot be sampled effectively. <br />-The age-class structure of the population can be estimated based on length-frequency <br />data. However, older year classes are harder to differentiate by this method as growth <br />rates decrease with age. <br /> <br />o Humpback chub: <br />-Adult humpbacks can be sampled relatively effectively and the size of their populations <br />can be estimated. <br />-Young and juvenile humpbacks cannot be sampled effectively. <br />-The age-class structure of the population can be estimated based on length-frequency <br />data. However, older year classes are harder to differentiate by this method as growth <br />rates decrease with age. <br /> <br />I <br />